The act in which a girlfriend seduces her boyfriend for sex. However, once inside her, the boyfriend realizes that he has entered a dark and bloody mess. Often accompanied by a foul odor and a girlfriend shouting "Boo!" Most often occurring around Halloween.

Last night was Halloween and my girlfriend was on her period and I didnt know. To make a long story short, she gave me a Halloween Surprise. Needless to say, it was a mess! :(